FIG. 1 shows a conventional ferroelectric memory cell 101 having a transistor 130 and a ferroelectric capacitor 140. The capacitor comprises a ferroelectric ceramic thin film sandwiched between first and second electrodes 141 and 142. Electrode 142 is coupled to a plateline 170 and electrode 141 is coupled to the transistor which selectively couples or decouples the capacitor from a bitline 160, depending on the state (active or inactive) of a wordline 150 coupled to the transistor gate. A plurality of cells are interconnected by platelines, bitlines, and wordlines to form an array.
The ferroelectric memory stores information in the capacitor as remanent polarization. The logic value stored in the memory cell depends on the polarization of the ferroelectric capacitor. To change the polarization of the capacitor, a voltage which is greater than the switching voltage (coercive voltage) needs to be applied across its electrodes. An advantage of the ferroelectric capacitor is that it retains its polarization state after power is removed, resulting in a non-volatile memory cell.
FIG. 2 shows a cross section of a conventional ferroelectric capacitor 140 on a plug 266. As shown, the capacitor comprises a ferroelectric layer 246 sandwiched between bottom and top electrodes 141 and 142. The electrodes typically are formed from a noble metal such as platinum. The bottom electrode 141 is coupled to the plug which, for example, is in contact with a diffusion region of the cell transistor. A barrier layer 264 can be provided below the lower electrode to protect the plug from oxidation. A dielectric layer 280 serves as the interlevel dielectric layer. The dielectric layer typically comprises TEOS. A contact 260 is provided, coupling the top capacitor electrode to, for example, a plateline.
During certain processes, hydrogen is present. The hydrogen can be part of the process or generated as part of the process (e.g., by product). For example, hydrogen is used as part of the ambient in forming gas anneals or generated as by-products in plasma TEOS or tungsten CVD deposition processes. The presence of hydrogen, however, is undesirable as it can degrade the properties of the ferroelectric materials of the capacitor. The degradation mechanism is mainly due to the reduction of the capacitor oxide layer affected by H—O bonding.
Conventionally, to reduce capacitor degradation or failure caused by hydrogen, an encapsulation layer 283 is provided. The encapsulation layer surrounds the top and side of the capacitor, protecting the capacitor from diffusion of hydrogen. The encapsulation layer comprises an insulating material which inhibits diffusion of hydrogen, such as silicon oxide or aluminum oxide. An insulating material is required to prevent shorting of the capacitor.
However, in order for the contact to be electrically coupled or have good contact characteristics, the portion of the insulating encapsulation layer is removed in the contact region. As a result, hydrogen can through the contact area to the capacitor during, for example, decomposition of TEOS resulting from contact opening formation, tungsten fill process to form the contact or forming gas anneals (typically formed after contact formation), thus degrading or causing failure to the capacitor.
From the foregoing discussion, it is desirable to provide a capacitor with improved resistance against the diffusion of hydrogen.